Swept wavelength lasers and wavelength interrogator systems employing such lasers are useful in a variety of communication and sensing applications.
Wavelength-division multiplexing (WDM) has greatly expanded the number of channels that can be carried on fiber optic systems. Swept wavelength laser systems capable of measuring and characterizing component channels spaced as narrowly as 50 GHz are desirable in current communication applications. WDM systems require wavelength and amplitude measurement techniques in both transmission and isolation paths. Preferred techniques provide rapid measurement over the large number of channels employed. A WDM component test system based on the use of a swept wavelength laser has been reported. (B. Nyman [1998] Optoelectronics World, September, pp. 527-532.)
Fiber optic sensor arrays incorporating fiber Bragg gratings are employed in a variety of engineering and environmental sensing applications. The wavelength reflected by a fiber Bragg grating is affected by environmental perturbations (e.g., strain, temperature) so that changes in the wavelengths of FBGs in an array can be used to detect these perturbations. FBG sensor arrays require wavelength interrogator systems to measure small changes in wavelength, preferably for large numbers of individual FBGs. Rapid wavelength detection is also preferred. S. H. Yun et al. (1998) Optics Letters 23(11):843-845, reports the use of a tunable swept wavelength laser interrogator system for use with FBG sensor arrays.
Swept wavelength laser systems and wavelength interrogators employing these lasers that have been described do not provide for wavelength calibration over the entire wavelength range to be scanned.
This invention provides swept-wavelength lasers with accurately calibrated wavelength output which can be very rapidly scanned or swept over a selected wavelength band. In particular, the invention provides lasers that generate wavelengths in the 1550 nm range that can be swept over about 50 nm. These swept-wavelength lasers are generally useful as accurately calibrated high power light sources. Calibration is achieved by use of a calibrated reference system. Swept-wavelength lasers are particularly useful as components of sensor interrogator systems which determine wavelengths reflected (or transmitted) by Fiber Bragg Gratings (FBG). Swept wavelength lasers of this invention are also generally useful for testing of WDM systems, particularly for their applications to current communication systems. This invention provides a method of calibrating a swept-wavelength laser using the reference system provided herein. Additional systems provided include various interrogators for sensor arrays and systems for testing the performance of WDM system components as well as methods of measuring wavelength(s) transmitted by an optical device and methods for testing the performance of WDM components.
The reference system employed herein for calibrating the swept-wavelength laser and which is employed as a component in sensor interrogation devices has been described in U.S. Pat. Nos. 5,838,437, 5,892,582 and co-pending U.S. patent application Ser. No. 09/286,411, filed Apr. 5, 1999, which are incorporated in their entirely by reference herein to the extent not inconsistent with the disclosure herein. The reference system employs fixed fiber Fabry-Perot filters which generate a comb of wavelengths and a reference FBG (FBGR) which reflects a known wavelength or transmits output having a notch at a known wavelength. FBG""s are well-known devices and their use in this wavelength reference has been described in the U.S. patent applications listed above. Fixed cavity length FFP filters have been described in the following U.S. patents all of which are incorporated by reference herein for their description of the construction and operation of such filters: U.S. Pat. Nos. 5,212,745; 5,212,746; 5,289,552; 5,375,181; 5,422,970; 5,509,093; and 5,563,973. All-fiber FFPs constructed in fiber ferrule assemblies are preferred for use in the lasers of this invention.
The swept-wavelength lasers of this invention are illustrated as employing tunable fiber Fabry-Perot filters (FFP-TF). These filters can be constructed to have high finesse and can be rapidly scanned or swept by changing the cavity length, for example, using piezoelectric transducers (PZTs). All-fiber FFP-TF""s constructed in fiber ferrule assemblies and having alignment fixtures that allow rapid tuning of cavity length without significant losses due to fiber misalignment are preferred. The construction and operation of FFP-TF are also described in U.S. Pat. Nos. 5,212,745; 5,212,746; 5,289,552; 5,375,181; 5,422,970; 5,509,093; and 5,563,973.